1. Field of the Invention
The present invention relates to a television tuner and in particular to a television tuner which is capable of receiving general television broadcasting, CATV, and satellite broadcasting.
2. Description of the Related Art
Recently, an advance in audio and visual technology has been drastic. An increase in the number of broadcasting channels and a further improvement in picture quality have been demanded. A development for providing more compact satellite broadcasting receiving systems and for incorporating the satellite broadcasting receiving system into a television system has become more active.
It is advantageous to adopt a double superheterodyne system for a TV tuner in consideration of multi-channel reception and necessary band pass characteristics for each channel.
A conventional double superheterodyne type television tuner will be described with reference to the drawings.
Referring now to FIG. 1, there is shown the structure of a conventional double superheterodyne type television tuner. In FIG. 1, reference numerals 1 though 3 denote input band pass filters, each having different frequency characteristics; reference numeral 4 denotes a broad band amplifier (RF amplifier); 5 denotes a first mixer; 6 denotes a first local oscillator; 7 denotes a first intermediate frequency (IF) amplifier; 8 a band pass filter (BPF); 9 a second mixer; 10 a second local oscillator; 11 a second IF amplifier and 14 a prescaler.
The operation of the thus formed tuner shown in FIG. 1 will now be described. A high-frequency signal is inputted into the input band pass filters 1, 2, and 3 provided for each band (for example VHF, CATV and UHF) through a terminal A. The signal which has passed through the filters 1, 2 and 3 is amplified by the broad band amplifier 4 and is then inputted to the first mixer 5. The signal is mixed with an oscillation signal from the first local oscillator 6 in the first mixer 5 from which a signal having a difference component therebetween is outputted as a first IF signal. The IF signal is then amplified. Only the first IF signal is selected by the band pass filter 8 and is then inputted to the second mixer 9 in which the inputted signal is mixed with a second oscillation signal from the second local oscillator 10. A signal having a difference component is outputted from the second mixer 9 as a second IF signal. The second IF signal is inputted to a terminal B through the second IF amplifier 11. The first local oscillator 6 is supplied with a local oscillation tuning voltage. The local oscillation signal from the first local oscillator 6 is frequency-divided by the prescaler 14 from which the frequency-divided output is fed to a channel selector circuit (not shown) from a terminal E.
In the afore-mentioned arrangement, any one of the first IF amplifier 7 and the band pass filter 8 may be preceded by the other according to the design.
The first half of the tuner before the band pass filter 8 in FIG. 1 will be referred to as an up-converter meaning a portion which converts the inputted signal frequency into an IF frequency higher than the inputted signal frequency. The latter half of the tuner after the band pass filter 8 in FIG. 1 is referred to as a down-converter meaning a portion which converts the first IF signal frequency into a second IF frequency lower than the first IF frequency.
A conventional example of a satellite broadcasting receiving tuner (referred to as a BS tuner) will be described with reference to FIGS. 2 and 3. FIG. 2 shows a portion of the BS tuner referred to as a BS second mixer portion comprising an input filter 21, a BS first IF amplifier 22, a mixer 23, a local oscillator 24, a BS second IF amplifier 25, and a prescaler 26. FIG. 3 shows an FM demodulator for the BS tuner comprising a BS second IF amplifier 30, a band pass filter 31, another BS second IF amplifier 32, a phase comparator 33, a loop filter 34, a direct current amplifier 35, a base band amplifier 36, and an oscillator 37.
The operation of the thus formed mixer and FM demodulator shown in FIGS. 2 and 3 will be described. In FIG. 2, a signal from a broadcasting satellite (1.0 to 1.3 GHz) is block-converted into the first intermediate frequency (referred to as BSIF) at a pre-stage, and the converted signal is inputted into a terminal C. The band pass filter 21 passes only a BSIF and inputs the BSIF signal to the mixer 23 through the BS first IF amplifier 22. The BSIF signal is mixed at the mixer 23 with an inputted signal from the local oscillator 24. The mixer 23 outputs a difference signal as a BS second IF signal having a frequency of 403 MHz to a terminal D through the BS second IF amplifier 25. A tuning voltage is applied to the local oscillator 24 from a terminal F' and the local oscillation signal is also applied to the prescaler 26 from the local oscillator 24. The applied signal is frequency-divided and applied to a channel selection circuit (not shown) from the terminal E'.
The BS second IF signal which has been inputted to the terminal D is then inputted to a terminal E in FIG. 3 and amplified in the BS second IF amplifier 30 and is passed though the 400 MHz band pass filter 31 and then inputted into an FM demodulator through the BS second IF amplifier 32. The FM demodulator comprises a phase comparator 33, a loop filter 34, a direct current amplifier 35, and an oscillator 37 which form a phase-locked loop (PLL) for PLL detection. The FM demodulated output from the FM demodulator is inputted to the base band amplifier 36 which provides an amplified demodulated signal to a terminal F.
A conventional example of the structure of a double superheterodyne tuner will be described with reference to FIGS. 4A, 4B and 5. FIGS. 4A and 4B show an arrangement of a housing for a double superheterodyne tuner and a satellite broadcasting receiving tuner, respectively. FIG. 4A shows an arrangement of the double superheterodyne tuner shown in FIG. 1 comprising an up-converter 50, an up- and down-converter coupling filter 51, a down-converter 52 and an antenna input terminal for VHF and UHF signals. FIG. 4B shows an arrangement of a satellite broadcasting receiving tuner shown in FIG. 2 comprising a BS second mixer portion 54, an FM demodulator 55 and a BSIF input terminal 56.
FIG. 5 shows the up- and down-converter coupling filter (up- and down-converter coupler) 51 forming a low pass filter comprising feed-through type capacitors 61 and 62 disposed on the up- and down-converter sides, respectively and a air-cored coil 63. On the both sides of the up- and down-converter coupling filter 51, there are disposed circuit boards 64 and 65 with respective shield plates 66 and 67 for the up- and down-converters. The air-cored coil 63 is disposed in a space between the shield plates 66 and 67. The opposite ends of the air-cooled coil 63 are soldered to the leads of the feed-through type capacitors 61 and 62 secured to the shield plates 66 and 67, respectively to form the filter. The purpose of the shield plates and the low pass filter is to prevent spurious signals from being generated at the down-converter output due to an interference between the local oscillation signals in the up- and down-converters.
In order to provide a tuner which is capable of receiving both a general television signal and a satellite broadcast signal, it is necessary to provide separate housings as shown in FIGS. 4A and 4B and to provide a space for a filter like the filter 51 in the television tuner as shown in FIG. 4A to prevent spurious radiation from being generated. A configuration like this makes it difficult to utilize the space effectively since circuit boards for up- and down-converters have to be separated from each other, and besides, it necessitates lengthy manufacturing steps.